1. Field of the Invention
The present invention relates to an electron emitting device, an electron source constituted by this electron emitting device and an image-forming apparatus which is an application thereof, and more particularly to a surface conduction electron-emitting device having a novel structure, an electron source using this device and an image-forming apparatus such as a display unit which is an application thereof.
2. Related Background Art
A surface conduction electron-emitting device utilizes such a phenomenon as that electron emission is generated by flowing an electric current through an electroconductive thin film formed on a substrate.
As an example of this surface conduction electron-emitting device, there are reported a device using an SnO2 thin film [M. I. Elinson Radio Eng. Electron Phys., 10, 1290, (1965)], a device using an Au thin film [G. Ditmmer, Thin Solid Films, 9,317 (1972)], a device using an In2O3/SnO2 thin film [M. Hartwell and C. G. Fonsted, IEEE Trans. ED Conf., 519 (1975)], a device using a carbon thin film [Hisashi Araki and et al: SHINKU (Vacuum), Vol. 26, No. 1, p. 22 (1983)] and others.
In these surface conduction electron-emitting devices, it is general to cause electron emission by performing an energization operation called “forming” with respect to the electroconductive film before carrying out electron emission.
Here, “forming” means that a fixed voltage or a voltage which slowly rises at a rate of, e.g., approximately 1V/min to both ends of the electroconductive film and an electric current is caused to flow through the electroconductive film so that the electroconductive film is locally fractured, deformed or transformed to have an electrically high resistance, thereby generating electron emission.
A fissure is formed on a part of the electroconductive film with this operation, and it can be considered that the phenomenon of electron emission occurs due to existence of this fissure. Although a position where the actual electron emission occurs is not completely cleared, the fissure and the surrounding area thereof may be referred to as “an electron-emitting region” for the sake of convenience.
The present applicant has advanced many suggestions concerning the surface conduction electron-emitting device. For example, in regard to the above-described “forming”, Japanese patent No. 2854385, U.S. Pat. No. 5,470,265, and U.S. Pat. No. 5,578,897 disclose that forming is preferably carried out by application of a pulse voltage to the electroconductive film.
Here, a waveform of the pulse voltage can be appropriately selected by any of a method for maintaining a wave height value constant such as shown in FIG. 5A and a method for gradually increasing the wave height value such as shown in FIG. 5B, taking into account a shape or form of the device and a condition for forming.
Further, it has been discovered that an electric current flowing through the device (device current If) and an electric current involved by electron emission (emission current Ie) are both increased by repeatedly applying the pulse voltage to the electron-emitting device in an atmosphere containing an organic substance after the above-described forming, and this process is referred to as “an activation operation”.
This operation forms a deposition containing carbon as a main component in an area including a fissure formed on the electroconductive film by “forming”, and its detail is disclosed in Japanese Patent Application Laid-Open No. 7-235255.
When the above-described surface conduction electron-emitting device is applied to the image-forming apparatus and the like, low-consumption power and high brightness are further required.
Therefore, as a performance of the electron-emitting device, a ratio of the emission current Ie to the device current If, i.e., the electron-emitting efficiency needs to be higher than that of the prior art.
In order to improve such a performance, it is naturally necessary that a variation in the performance with time due to continuation of the electron emission is not larger than that in the prior art.